SECT. II 



PHYSIOLOGY 



193 



hausted by thcni. The system of branches of one root system often lias an 

 astonishingly great total length. 



All plants do not form a deep-growing tap-root like that of the Oak, Silver Fir, 

 Beet, Lucerne, etc. ; some confine themselves to utilising the superficial layers of 

 the soil by means of a thickly-branched lateral root system (Pine, Cereals). The 

 agriculturist and forester must, accordingly, take into consideration the mode of 

 branching and growth of the roots of a plant, just as much as the habit of growth 

 of its aerial portions. Plants which make use of different layers of soil may lie 

 safely cultivated together in the same soil, and succeed one another in the same 

 ground. For similar reasons, in setting out trees along the borders of fields, the 

 deep-rooted Elm should be preferred to the Poplar, whose roots spread out near 

 the surface. 



Desert plants send out deeply penetrating roots, which only branch profusely 

 on reaching depths where they find water. When grown in moderately dry soil 

 the growth of the roots of oats is greater than when the soil is damp. 



Movement of Water in the Plant 



That a movement of water from the roots to the aerial parts of the 

 plant must take place follows from the fact that water is required, both in 

 the construction of carbohydrates and in the development of new cells 

 in the growing regions. The plant, however, requires far more water 

 than is needed for its construction, because it gives off large quantities 

 of water in the form of vapour, and a less amount in the liquid form 

 from its aerial portions. The former process is known as transpira- 

 tion, the latter as EXUDATION. 



Tpanspiration (^) 



The vegetable cell, like every free surface of water or substance 

 swollen with water {e.g. gelatine, mucilage), must give up water to the 

 air so long as the latter is not completely saturated. Under certain 

 conditions the loss of water from some parts of plants {e.g. roots, sub- 

 merged portions, shade plants), is very great. Such objects exposed to 

 dry air, especially in the sun, lose so much water that they become 

 collapsed, limp, and wilted, and ultimately dried up. The leaves borne 

 on ordinary land plants behave otherwise. At first sight no loss of 

 water is perceptible from them ; but they also wilt during a drought, 

 which renders absorption of water from the soil difficult. If the supply 

 of water to them is interrupted more suddenly, as by cutting them 

 off, the wilting occurs more speedily. That they as a rule do not wilt 

 when in position on the plant evidently depends on the fact that water 

 is supplied from below in equal amount to that evaporated from above. 

 The giving-off of water can be demonstrated by simple methods. 



If a transpiring part of a plant is covered with a bell-jar that has been })reviously 

 cooled, the water vapour given off from the plant will be deposited in the liquid form 

 on the inside of the bell-jar, just as the aqueous vapour in our breath condenses on a 



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